A 10G-EPON (Gigabit Ethernet (registered trademark) Passive Optical Network) system has been known which connects a plurality of optical network units (ONU) installed in subscriber's homes to an OLT installed in a station through optical fibers (see Non-Patent Document 1, for example). In such a 10G-EPON system, a first ONU transmits an optical burst signal to the OLT at the transmission rate of 1.25 Gbps (Gigabit per sec), and a second ONU transmits an optical burst signal to the OLT at the transmission rate of 10.3125 Gbps. Since the optical burst signals with the different transmission rates have wavelength ranges overlapping with each other, the optical burst signals from a plurality of ONUs are time division multiplexed before transmission to the OLT.
As for optical receivers in the OLT to which the optical burst signals are input, they are required to have appropriate reception sensitivity for a plurality of optical burst signals with different transmission rates. Therefore conventionally, the conversion gain of a preamplifier circuit constituting each optical receiver is controlled in accordance with the transmission rate of the optical burst signal to achieve the appropriate reception sensitivity corresponding to each transmission rate (see Patent Document 1, for example).
On the other hand, it is necessary for the OLT to positively detect the disconnection when the input of the optical burst signal is disconnected. To achieve this, if the input disconnection is detected, the optical receiver must accurately output the input disconnection signal indicating that the input is disconnected. The input disconnection signal is output when the amplitude of the output signal of the preamplifier circuit is smaller than a preset threshold, and the OLT detects erroneous light emission of the ONU according to the presence or absence of the input disconnection signal.